Some points :
Its the traditional spell, the one that bonisagus used so it's the proper way to do it.
Medieval towers due to sheer mechanical strains usually are conical in shape (wider and the bottom than the top AND due to the way physics work, the lower you are in the tower the thicker the walls have to be just to bear the weight of what is above.
This castle has a perfect example of an old medieval donjon : chateaudemontrottier.com/
If you check this web site, English then Tour section, you will see on the bottom left a picture of the actual donjon.
Historically this castle was built at the end of the 10th century . On the same picture showing the keep, the first building to be built is, if i'm not mistaken, the one on the left. It did not even have a door on the first floor, when people wanted to enter it they climbed a ladder, which was then brought inside the castle. If someone was in, there was no way to climb to the door. Then the other parts, including the Keep, were built. Well that's what I remember from the guided tour.
(a source says : built by the Abbaye de Savigny, thus the religious usage and name of the first building).
That castle's donjon is conical, that is base is wider than the top, the hallway to enter it is on the second floor (something like 6 meters above ground) and is less than 2 feet wide. The visit is actually forbidden to overweight people. When you do visit it, everyone goes up, then everyone goes down at the same time. You can't cross inside the stairway, it's too tight.
Some numbers : 30.6 meters wide at the bottom, 33.5 meters high... That's a lot of stone. It was probably built during the 13th century, my memory of the tour is a bit fuzzy.
To come back to the spell : in my opinion, the bonisagus tower does not have to follow medieval architecture rules. There is a simple reason to that : it is created as it is instantly, thus it avoids the complication of having to build it over 10 or 15 years, just because you have to bring those 27000 cubic feet of stones where you want your donjon to be... And also, the magic knows statics, thus a "perfect" building could be built by the magic of the spell, allowing a building having lines closer to the 20th century than suspension of disbelief would bring us to think of.
(I don't know the english term for Statique, I tried statics, but it sounds weird and wrong)
I think that this spell creates a round stone building, which follows modern rules of construction, as the statics are perfectly defined (unless the spellcasting is botched of course). The main material is PERFECT stone, without cement or cracks, even though it appears to have cement and cracks from the outside (remember a craftman has to make a stonemasonry check vs finesse to detect it is made by magic). Thus as it is perfect stone, it is also perfectly balanced, allowing THIN walls. I even allow stone ducts to allow air flow between floors. At least that's how I see it. Magic is not engineering, the fact that the creator of this spell knew Stone through his mastery of Terram allowed him to break the limits of the time and build something which his cotemporary architects could not even imagine.
That's of course if in MY game 
'cause our Bonisagus needs to know how much stone will be needed before he starts inventing the spell.